Журнал эффектов и контроля загрязнения
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Heavy Metals in an Important Section of the São Francisco River (Northeast Brazil): Distribution Profile, Accumulation Mechanisms, and Risks of Dissemination through the Food Chain

Pereira MDG*, Souza CLM, Sachdev RDL, Santos AVD, Pinto PADC, Souza LA, Ribeiro JN, Oliveira JPD, Guimarães MCC and Ribeiro AVFN

Background: This work assessed the distribution of Cd, Cr, Cu, Mn, Ni, Pb, and Zn in an important Brazilian fluvial section, which has never been evaluated before this study

Procedures: Along six samplings, total and bioavailable concentrations of these metals were quantified in water and sediments, respectively, by atomic spectrometry. Bioavailable metal concentrations were extracted from sediments by 0.1 molL^-1 HCl, while total metal concentrations were released in water after acidic decomposition with 14 molL^-1 HNO₃. Sediments were submitted to analyses of X-ray diffractometry, thermogravimetry, infrared spectrophotometry, electronic scanning microscopy, granulometry, and density. Subsequently, adsorption tests were performed with those elements that tend to exist as cations in river water (Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺).

Results: Water pH (6.9 to 8.3) favored precipitation of Cr and Mn, explaining their undetectable concentrations in water. Although cadmium, copper, lead, nickel, and zinc are stabilized as cations at water pH and chemical oxygen demand (<0.5 mgL^-1), total concentrations of these metals in water were undetectable. Sediments presented prevalence of quartz, sandy granulometry, maximum density of 2.8 gcm^-3, and remarkable bioavailable concentrations of Cr (up to 778.4 ± 37.4 mg kg^-1) and Mn (up to 230.9 ± 6.2 mg kg^-1, which can be readily assimilated by benthic organisms. Bioavailable concentrations of Cd, Cu, Ni, Pb, and Zn in sediments were considerable small (≤ 43.4 ± 2.5 mgkg^-1). Although sediments are limited for adsorbing Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ (≤ 1.49 mgg^-1), these cations are adsorbed with low reversibility, being progressively accumulated and disseminated through the food chain over a long period of time.

Conclusions: This work improved the knowledge about the distribution of heavy metals in fluvial environments, as well as evaluated the risks of an eventual contamination of humans.